1. Field of the Invention
The present invention relates to a developing device to be used for electrostatic charge image development in electrophotography, and an image forming apparatus and a process cartridge using the developing device.
2. Description of the Related Art
In image forming processes such as electrophotography and electrostatic photography, a developer that is obtained by mixing and stirring toner and carrier is generally used for developing a latent electrostatic image formed on a latent image bearing member. The developer is required to be a mixture having been charged.
There are generally known two methods for developing latent electrostatic images: a method using a two-component developer obtained by mixing toner and carrier; and a method using a one-component developer not including carrier. The former developing method using a two-component developer enables obtaining relatively stable and excellent images, while it has a drawback that carrier deterioration and a fluctuation in the mixing ratio of the toner and carrier are likely to occur. On the other hand, the latter method using a one-component developer does not have such a drawback as in the former method but has an inconvenience that the electrostatic property is hard to stabilize.
When repeatedly carrying out development of latent images using a two-component developer, toner in the developer is consumed to cause fluctuation in toner concentration, so that for obtaining stable images at the time of printing, it is necessary to replenish the developer with toner according to necessity so as to suppress the fluctuation.
As a method for controlling the amount of toner replenishment, copiers are generally equipped with permeability detecting sensors, fluidity detecting sensors, image density detecting sensors, bulk density detecting sensors, and the like, and recently, image density sensors have been predominately used. These sensors use a system for controlling the amount of toner replenishment by developing a regular image pattern on the latent image bearing member and detecting image density from reflected light.
For a particulate carrier to be used in such a two-component developing system, a hard and high-strength coating layer has been provided, usually, by coating with a suitable resin material for the purpose of preventing toner filming onto the carrier surface, forming a uniform carrier surface, preventing surface oxidation, preventing a decrease in moisture sensitivity, extending the service life of the developer, protecting the photoconductor from scratches or wear caused by the carrier, controlling charge polarity, adjusting the charge amount, or the like.
As examples of such carriers according to related arts, disclosed is a carrier in which the core surface is coated with a particular resin material (refer to Japanese Patent Application Laid-Open (JP-A) No. 58-108548), carriers in which various additives are added to the coating layers (Japanese Patent Application Laid-Open (JP-A) Nos. 54-155048, 57-40267, 58-108549 and 59-166968, Japanese Patent Application Publication (JP-B) No. 01-19584 and 03-628, and JP-A No. 06-202381), a carrier for which an additive is adhered to the surface (JP-A No. 05-273789), and a carrier in which the coating layer includes conductive particles larger in diameter than the thickness of the coating layer (JP-A No. 09-160304).
There are also known methods: in which a coating material whose main component is a benzoguanamine n-butyl alcohol formaldehyde copolymer is used for a carrier (see JP-A No. 08-6307); and in which a crosslinked acrylic resin and melamine resin is used as a coating material (Japanese Patent (JP-B) No. 2683624).
However, the carriers according to the related arts mentioned above are still insufficient in durability, and there is a problem such as a decrease in resistance due to scraping of the coating layer. More specifically, although satisfactory images can be initially obtained, the quality of copy images degrades as the number of copies increases, so that a further improvement has been demanded.
Therefore, a method has been proposed for solving these problems by dispersing, in the coating layer of a core, fine particles having a particle diameter equal to or more than the thickness of the coating layer (JP-A No. 2001-188388).
However, in this proposal, while the durability is improved, a problem of a rise in resistance of the developer on the magnetic brush occurs. If the carrier resistance rises, counter charge remaining in the carrier after development becomes hard to dissipate, so that carrier adhesion to a solid edge portion occurs. In an image portion, an electric field where the toner transfers from the developing sleeve surface toward the photoconductor drum is formed. In a non-image portion, an electric field where the toner transfers toward the photoconductor drum disappears.
Moreover, in an edge portion being a boundary between the image portion and the non-image portion, an edge electric field being an electric field where the carrier adheres to the photoconductor drum to the contrary is formed. The intensity of the edge electric field becomes stronger at a higher carrier resistance, and becomes weaker, at a lower carrier density. Therefore, usually used is one whose coating layer contains a conductive substance for the purpose of resistance control. However, contact probability of the coating layer decreases because of the existence of particles having a larger particle diameter than the thickness of the coating layer, and a sufficient effect of the conductive substance cannot be obtained. The resistance of the developer therefore rises.
Moreover, the contact area between the sleeve and the carrier also decreases, and thus counter charge becomes harder to dissipate.
In order to solve these problems, there has been proposed a method for lowering the resistance of a developer by providing a conductive coating layer containing fine particles having a particle diameter equal to or more than the thickness of the coating layer (JP-A No. 2006-184891). This proposal has revealed that the edge effect can be suppressed. However, the proposed method is not sufficient to dissipate counter charge by itself.
Meanwhile, among a large number of conventional arts relating to electrophotographic developer technologies, there are relatively few arts that disclose particularly preferred correlations between the surface condition of carrier particles and the surface roughness of the developing sleeve. For example, the inventor of the present application has before proposed an image forming apparatus in which the developing sleeve of the two-component developing device has a surface roughness (Rz) of 5 μm to 20 μm, the magnetic carrier has a coating layer including a binder resin and particles, the particles have a diameter D and the coating layer has a thickness h where 1<D/h<10, and the magnetic carrier has a weight average particle diameter d of 20 μm to 60 μm (JP-A No. 2003-228240). However, “d” of d/Rz provided in this proposal means an average particle diameter d of the carrier particles, while “D” of Ra/D of the invention of the present application means a particle diameter D of the conductive fine particles in the coating layer. Moreover, this proposal describes the “sleeve surface roughness Rz” and the “particle diameter D of the conductive fine particles,” but does not describe the “sleeve surface roughness Ra,” so that an Ra value cannot be strictly determined from an Rz value. Furthermore, the proposed technology aims to stabilize the scooping amount of the developer, while the invention of the present application aims to provide a developing technology for images excellent in durability while suppressing image defects such as carrier adhesion, and therein fundamentally differs therefrom. Moreover, in the examples of the proposal, only Experiment 4 describes both of the particle diameter of the conductive fine particles and the surface roughness of the developing sleeve, however, there exist only examples where D/h is 2.0 or more.
Therefore, the proposal of JP-A No. 2003-228240 does not reveal any features of the invention of the present application.